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By George JahnAssociated Press • Sunday December 9, 2012 11:10 AM

EDITOR’S NOTE: The following story clarifies a previous Associated Press story about leaked documents related to Iran’s nuclearprogram. The originalstory appeared on Page A1of the Nov. 28
Dispatch.
VIENNA — A leaked diagram suggesting that Iran is pursuing a nuclear weapon is scientifically flawed, diplomats working with the U.N. nuclear agency conceded.However, they insisted that it still supports suspicions that Tehran is trying to build a bomb, especially when combined with other documents that remain secret.

The Associated Press reported on Nov. 27, citing the document leaked by officials from a country critical of Iran’s atomic program, that it indicated that Iranian scientists had run computer simulations for a nuclear weapon that would produce more than triple the explosive force of the World War II bomb that destroyed Hiroshima.

The diagram showed a yield of 50 kilotons. But subsequent criticism of AP’s report showed that result was widely inaccurate. Instead, the yield of the hypothetical weapon was much higher and hugely greater than any bomb ever produced — meaning it was next to impossible that Iran was contemplat-ing such a weapon.

Nuclear scientists Yousaf Butt and Ferenc Dalnoki-Veress, writing on the website of the Bulletin of Atomic Scientists, sharply criticized the report and described it as containing a “massive error,” adding that it was “either slipshod analysis or an amateurish hoax.” They said the “level of scientific sophistication” to produce the diagram “corresponds to that typically found in graduate or advanced undergraduate-level nuclear-physics courses.”

But a senior diplomat familiar with the probe of Iran by the IAEA told the AP on Nov. 30 that the agency suspects that Iranian scientists calculating a nuclear yield intentionally simplified the diagram to make it comprehensible to Iranian government officials. He said that when the right data are plugged in, the yield is indeed 50 kilotons. The diplomat, who is considered neutral on Iran’s nuclear program, spoke only on condition of anonymity because he was not authorized to divulge intelligence.

As seen by AP, the data on the left-hand vertical side of the diagram is listed in kilotons per second. But David Albright, whose Institute for Science and International Security is used by the U.S. government as a go-to source on Iran’s nuclear program, told AP that when that legend is substituted with another — joules per 10 nanoseconds — the yield comes out to about 50 kilotons.

When told of Albright’s calculations, the senior diplomat confirmed that the agency thought they were correct. He also said the agency had a spreadsheet thought to have been drawn up by Iranian nuclear scientist Majid Shahriari that contained the data needed to produce a nuclear yield of 50 kilotons. He said Shahriari is also thought to have produced — and then altered — the diagram, which he said was one of several held by the IAEA showing such yield calculations.

Iran denies any interest in nuclear weapons and accuses the United States and Israel of fabricating evidence that suggests it is trying to build a bomb.

The senior diplomat said agency investigators realized the diagram was flawed shortly after they received it last year, but they think it remains important as a clue to Iranian intentions.

He and a second diplomat said other classified material held by the IAEA supports concerns that the graph might be part of a past Iranian effort at developing nuclear weapons. The second diplomat comes from a country suspicious of Iran’s nuclear intentions but not from the nation that shared the diagram with the AP. He too spoke only on condition of anonymity because he is not authorized to disclose secret information.

The diagram was disclosed in an attempt to bolster arguments that Iran’s nuclear program must be halted before it makes a weapon. The officials who leaked it provided what they described as a computer model of blast calculations only on condition that they and their country not be named.

In its report, AP said the curve of the diagram peaks at just above 50 kilotons at around 2 microseconds, reflecting the full force of the weapon being modeled. The bomb that destroyed the Japanese city of Hiroshima during World War II had a force of about 15 kilotons.

But experts said that if the figures and units on the diagram were correct, the blast produced would be nearly 2 million kilotons — far greater than the graph shows. It also would dwarf the biggest nuclear weapon ever built: the Soviet Union’s “Czar Bomba” detonated during a 1961 test with a yield of 57,000 kilotons.

Scott Kemp, a professor of nuclear engineering at MIT, called the 2-million-kiloton figure “impossibly high.”

“The numbers don’t add up to realistic weapons values; the axes do not agree with the caption,” he told AP. “At best ... this is an oversimplified theoretical simulation, not a serious model of an actual weapon.”

Dalnoki-Veress, one of those who originally questioned the accuracy of the diagram, told AP that the diagram does not make sense unless its creators “are thinking of an impossibly massive bomb.”Initial bomb tests of countries developing such weapons are usually in the 1- or 2-kiloton range.